The purpose of the proposed project is to test the hypothesis that pulmonary edema plays a central role in the pathogenesis of hyaline membrane disease. Lung water content, pulmonary blood flow and left atrial pressure measurements are being observed in newborn lambs with hyaline membrane disease in comparison with normal lambs of similar gestational age. In addition, lung water compartments in newborn rabbit pups will be measured at different fetal and postnatal ages by lung morphometry, by estimating the vascular space by using a Cr51-tagged red cell technique, and by estimating the non-vascular extracellular space from lung chloride.